Relays are electrical switching devices that use the flow of a first current to control the flow of a second current. A relay normally comprises two primary components: (1) an electromagnetic coil for generating a magnetic field based on the flow of the first current; and (2) a magnetically actuated electrical switch for controlling the second current, wherein the switch is actuated by the generated magnetic field.
Electromagnetic relays with electrical contacts are commonly comprised of a working gap that connects and disconnects the contacts, and an electromagnetic coil which produces a magnetic field that couples to the working gap via a magnetic path. To provide efficient coupling between coil and the working gap a readily magnetized or “soft” ferromagnetic material may be employed in the magnetic path. Further improvement in coupling is obtained when the soft ferromagnetic path is compact and consequently short with large cross sectional area. The force exerted on the relay contacts due to the magnetic field produced by the electromagnetic coil is a function of the material used in the device, the geometry of the coil, the number of turns in the coil itself, and the magnitude of the first current. Typically, the coil includes a large number of turns to keep the magnitude of the first current small.
In recent years, new microfabrication technologies, such as Micro-Electro Mechanical Systems (MEMS) technology, have been applied to the fabrication of relays. MEMS technology is based on planar processing operations that were first developed for use in the integrated circuit industry; however, MEMS technology affords the ability to form structures that are movable relative to their substrate. MEMS technology enables the fabrication of micro-relays that have several advantages over their macro counterparts, such as smaller size, lower cost due to the use of low-cost batch manufacturing, and new device functionality and applications that are enabled by their small size.
Prior-art micro-relays employ switches based on mechanically active switching elements such cantilever beams, doubly supported beams (i.e., bridges), plates, and membranes. These moving structures typically comprise a movable magnetic element comprising a first electrical contact. A magnetic field is applied to the magnetic element, which moves the first electrical contact into, or out of, contact with a second electrical contact (or pair of contacts) to enable or disable the flow of the second current.
Vertically actuated micro-relays comprise magnetic elements whose motion is enabled in a direction that is perpendicular to its underlying substrate. The creation of the movable structure in such a configuration is relatively straight-forward using conventional MEMS-based planar processing techniques. Using planar processing to add an efficient magnetic circuit having a compact magnetic path and large cross section area to such a structure is a challenge, however. In addition, the operating characteristics of such relays are primarily determined by the thin-film properties of the layers from which the movable magnetic elements are formed. The mechanical properties of thin-film layers can vary significantly depending on deposition conditions, however. Such variation can result in inconsistent operating characteristics even among micro-relays of the same design.
Laterally actuated micro-relays comprise magnetic elements whose motion is enabled along a plane that is substantially parallel to its underlying substrate. The magnetic element is typically supported above the substrate by tethers designed to be resilient for in-plane (i.e., lateral) motion but stiff for out-of-plane (i.e., vertical) motion. The tethers and magnetic elements are defined by photolithography and etching to “sculpt” them into their desired shape. Such micro-relays avoid some of the problems associated with vertically actuated micro-relays. In particular, the operating characteristics (e.g., resiliency, actuation force, operating speed, etc.) of a laterally actuated micro-relay depend more upon the defined structure of its tethers than upon the thin-film properties of the layers from which they are formed. As a result, the operating characteristics are substantially decoupled from deleterious effects due to film stress, thickness variations, and the like.
Typically, it is most desirable to use an electromagnetic coil to control the magnetic field that actuates a micro-relay, whether the magnetic field is generated by a permanent magnet or by the electromagnetic coil itself. Implementing an electromagnetic coil within a batch wafer-level process can be quite challenging, however, due to the three-dimensional character of such a coil and the need to efficiently magnetically couple it to the movable magnetic element. Thus, unfortunately, it is difficult at best to produce a practical integrated coil that can reliably actuate these switching elements.
As a result, micro-relays in the prior art have typically relied upon poorly coupled coils or external, non-integrated coils to provide the magnetic field for actuation. With a poorly coupled coil, however, the consequent large electrical power required to energize the relay is a significant drawback. The use of an externally configured coil not only adds significant packaging cost and size, but typically poor assembly tolerances can lead to significant variation in the operating characteristics of micro-relays of the same design.